The challange of finding books sophisicated enough keep their interest and yet not too "adult" in content led me to Terry Brookes and the Sword of Shannara series
LOVE the Shannara, esp. to give teenage boys. David hadn't read a book voluntarily in his LIFE until my mother made me let him have the copy of Sword that I took with us on a 7 week long RV trip we took across the whole US and back. He was forced by sheer boredom to start reading it, and never looked back as a reader.
Also give Brooks' Landover series a try: it starts with
Magic Kingdom for Sale--Sold! I'm sure all your teens will love the scribe who's been turned into a large dog by the inept wizard.Â
Also quite good for avoiding most objectionable adult material is David (and oft-uncredited Leigh!) Eddings Belgariad/Mallorean series. Some of their other work isn't as teen friendly, but these were sort of designed as YA fantasy novels.Â
About James Herriott: Just keep in mind that there IS a good bit of fairly explicit 'vet' work! The opening scene of
All Creatures Great and Small has the poor guy on the floor of a cow shed, stripped to the waist and up to his armpit trying to sort out a breech position calf, remember!
 Be prepared for some birds and bees explaining with these. I gave a whole set of Herriott to Desi's kids last summer (11 and 12) and trust me, there was a WHOLE lot of discussion about just where that guy had his arm, what he was doing, why it was necessary, and did that ever happen to human moms, etc. Herriott can be a great read for kids, but be sure yours is ready for the gory details.Â
And don't forget Anne McCaffrey. The Dragonriders trilogy may be a little old for early teens (there's some stronglly implied passion going on when those male dragons fly the queens!) but the Harper Hall books were deliberately written as a YA intro to the world of Pern.